1. Field of the Invention
The invention relates in general to a zoom lens system and more specifically but not exclusively to a zoom lens system used in optical photographic systems.
2. Description of the Related Art
Recently, image pickup apparatuses, such as video cameras and digital still cameras, including solid-state imaging devices, such as a charge-coupled device (CCD) sensor, have become more sophisticated and smaller in size. Therefore, a photographic optical system included in such a sophisticated and small-sized image pickup apparatus must also have high optical performance and be small in size.
A zoom lens included in a conventional image pick up apparatus has a four-unit structure comprising a first to a fourth lens unit disposed in order from the object side to the image side of the zoom lens system. The first lens unit has a positive refractive power and does not move during zooming and focusing. The second lens unit has a negative refractive power and moves along the optical axis for magnification. The third lens unit has a positive refractive power and does not move during zooming and focusing. The fourth lens unit has a positive refractive power and moves during zooming and focusing. (Refer to Japanese Patent Laid-Open Nos. 7-270684 (corresponding to at least one exemplary embodiment of U.S. Pat. No. 5,963,378), 7-318804 (corresponding to U.S. Pat. No. 5,963,378), 8-292369 (corresponding to U.S. Pat. No. 5,940,221), and 11-305124 (corresponding to at least one exemplary embodiment of U.S. Pat. No. 6,166,864).
In such a four-unit zoom lens, the first lens unit comprises a cemented lens, formed by bonding a negative/positive lens combination with a positive lens. The second lens unit comprises two negative lenses and a positive lens. The third lens unit comprises one or two positive lenses and a negative lens. An aperture stop is disposed immediately before the third lens unit.
Because recently the size of image pickup apparatuses has been reduced and pixel size has been minimized due to high resolution solid-state imaging devices, photographic optical systems used for such apparatuses are required to have high optical performance with a small zoom lens system having a short overall length. Many optical photographic systems (e.g. video cameras) are subject to the same high-quality still image recording requirements, thus a lens system having improved optical performance would be useful in such optical photographic systems (e.g. video cameras).
Typically, the overall size of a zoom lens system can be reduced by increasing the refractive power of the lens units because, in this way, the lens units will be required to move shorter distances in order to obtain a predetermined zooming ratio.
However, when the refractive powers of the lens units are simply increased, the variation in aberration that occurs during zooming also increases. Thus, it becomes difficult to obtain satisfactory optical performance for the entire zooming range. Furthermore, the required degree of accuracy in the production process increases, and mass production becomes difficult.